The broadcast nature of wireless mediums can be susceptible to threats regarding privacy and security of wireless communication. For example, an eavesdropper may be able to capture wireless signals. Depending on the type of wireless transmission, these signals can be available from a variety of distances, for example several yards, a mile, or across the globe. Data protection techniques can provide one form of security to communication. Encryption with a secret key is one method to conceal data. However, encryption techniques may not be sufficient for some scenarios, such as securely sharing a secret key with the intended nodes. Therefore, alternative means are needed to increase security. For example, physical layer security can provide enhanced security in the waveform domain. Security measures dealing with the waveform domain can prevent the extraction of the actual data properly. An eavesdropper can be prevented from extracting the actual data even if they can receive entire transmissions.
A multiple-input multiple-output (MIMO) system can have enhanced physical security by the addition of artificial noise to the actual signal. The noise signal can be designed to fall in the null space of the channel matrix between the antennas of legitimate devices using the MIMO system. Artificial noise can be eliminated while the signal is passing through the channel. Another device located in a different place has a different channel matrix than a legitimate transmitter. The artificial noise can distort the actual signal. The distortion of the actual signal can increase security. In a single-input single-output system (SISO) for single carrier-frequency domain equalization (SC-FDE) waveforms, artificial noise can be added to fading frequencies. For example, the artificial noise can be added to the fading frequencies instead of adding artificial noise to the null space. Fading frequency information can only be determined by legitimate transmitters. Thus, an eavesdropper cannot separate the actual signal and artificial noise. As such, an eavesdropper can experience a significant error. However, unlike SC-FDE, where data symbols are carried in the time domain, OFDM systems carry data symbols in the frequency domain. Therefore, introducing a noise signal to an OFDM signal might not prevent eavesdroppers from detecting which frequencies are utilized and ignoring transmissions on those frequencies since noise has a different characteristic compared to the data.
In wireless communication systems, orthogonal frequency division multiplexing (OFDM) can provide numerous advantages, for example OFDM can transmit data with high bandwidth efficiency, OFDM can be implemented with fast Fourier transformation (FFT), and OFDM can be equalized simply by exploiting the advantage of cyclic prefix (CP). On the other hand, OFDM signals can suffer from high peak to average power ratio (PAPR) caused by the parallel data transmission because of the non-linear characteristics of power amplifiers. At the output of such an amplifier OFDM signal can be nonlinearly scaled, and in-band interference can occur at the transmitter.
The drawings illustrate only example embodiments and are therefore not to be considered limiting of the scope described herein, as other equally effective embodiments are within the scope and spirit of this disclosure. The elements and features shown in the drawings are not necessarily drawn to scale, emphasis instead being placed upon clearly illustrating the principles of the embodiments. Additionally, certain dimensions or positionings may be exaggerated to help visually convey certain principles. In the drawings, similar reference numerals between figures designate like or corresponding, but not necessarily the same, elements.